As ultraviolet rays can cause skin cancer, it is undesirable to get sunburned heavily. Recently, health equipment (a sunburn alarm) which measures the intensity of ultraviolet rays and notifies the user of the degree of sunburn has appeared on the market. In accordance with the intensity of ultraviolet rays measured with the sunburn alarm, he/she selects the appropriate type of sunscreen or decides to stay indoors on days when ultraviolet rays are extremely strong.
Among the types of existing light sensors are solar batteries and photodiodes. They are a crystalline unit which uses a semiconductor, such as silicon, GaAsP, or GaAs as basic materials. Currently, the unit is manufactured in the form of a thin disk called a “wafer”, 20 to 30 cm in diameter, and undergoes such processes as fitting electrodes so as to be a sensor. As the size of one sensor is roughly 1 to 3 mm2, 15,700 sensors of 2 mm2 diameter can be produced from a 20 cm diameter disk. As a fabrication plant considers 25 disks as a lot, the size of one lot is as large as 392,500 units. Therefore, if much smaller quantities are needed, production is difficult and expensive.
Both silicon and GaAs, in pure crystal form, are insulators and do not conduct electricity. They also do not provide a photoelectric response. To enable these elements to provide a photoelectric response as a semiconductor, a lattice defect, or an electron hole, must be generated in the crystalline unit. Therefore, trace amounts of impurities must be added to the silicon or GaAs.
In crystal-based light sensors, the material added to the crystal could be arsenic or gallium, which are not safe. In lower priced light sensors, cadmium sulfide is used as the resistive element; however, cadmium sulfide is toxic and undesirable for widespread use. In addition, cadmium sulfide is only sensitive to specific wavelengths of visible light, and is not capable of having its wavelength sensitivity readily adjusted, thus its usefulness is limited. Further, because the photoelectric effect of crystal-based light sensors converts incoming light into electric current, analog electronic circuits are needed to amplify these minute electric currents, raising them to levels where other circuits can process them. Electricity consumption then becomes problematic and consistent operation becomes difficult in battery driven instruments.